1. Field of the invention
This invention relates to light interference filters for lamps, and is directed more particularly to a method for making tantala/silica interference filters and to an electric lamp having such a filter thereon.
2. Description of the Prior Art
Thin film optical coatings, known as interference filters, which comprise alternating layers of two or more materials of different indices of refraction, are well known to those skilled in the art. Such coatings, or films, are used to selectively reflect or transmit light radiation from various portions of the electromagnetic radiation spectrum, such as ultraviolet, visible and infrared radiation. The films or coatings are used in the lamp industry to coat reflectors and lamp envelopes. One-application in which the thin film optical coatings are useful is to improve the illumination efficiency, or efficacy, of incandescent lamps by reflecting infrared energy emitted by a filament, or arc, back to the filament or arc while transmitting the visible light portion of the electromagnetic spectrum emitted by the filament. This lowers the amount of electrical energy required to be supplied to the filament to maintain its operating temperature. In other lamp applications, where it is desired to transmit infrared radiation, such filters reflect the shorter wavelength portions of the spectrum, such as ultraviolet and visible light portions emitted by the filament or arc, and transmit primarily the infrared portion in order to provide heat radiation with little or no visible light radiation. Such an application of this latter type includes a typical radiant heater for use where visible radiation emitted by the heater is unwanted.
Such interference filters useful for applications where the filter will be exposed to high temperature in excess of 500.degree. C., or so, have been made of alternating layers of tantala (tantalum pentoxide Ta.sub.2 O.sub.5) and silica (SiO.sub.2), wherein the silica is the low refractive index material and the tantala is the high refractive index material. Such filters, and lamps employing same, are disclosed in U.S. Pat. Nos. 4,588,923; 4,663,557 and 4,689,519. In such lamp applications, the interference filters, which are applied on the outside surface of the vitreous lamp envelope containing the filament within, often reach operating temperatures in the range of about 800.degree. -900.degree. C. These interference filters, or coatings, have been applied primarily using evaporation or sputtering techniques which, while capable of producing a satisfactory interference filter, have limitations with respect to not being able to apply a uniform coating to any but a flat surface. In the case of tubing used for making lamps, the tubing must be rotated in the sputtering or vacuum evaporation chamber as the coating is being applied. This technique does not lend itself to applying uniform coatings to curved objects. Moreover, this technique is rather costly.
In U.S. Pat. No. 4,949,005, issued Aug. 14, 1990, in the name of Thomas G. Parham, et al, there is described a method for the manufacture of thin film interference filters consisting of alternating layers of tantala and silica suitable for high temperature use on electric lamps. Depending upon the individual layer thickness, such filters may be designed to reflect light with wavelengths falling within a particular range, while transmitting light of other wavelengths. As described in the '005 U.S. Pat. No., one application of such thin film interference filters is as coatings on the vitreous envelopes of incandescent lamps, which coatings improve lamp efficiency by reflecting infrared energy emitted by the lamp filament back onto the filament, while transmitting visible light emitted by the filament. The method for the manufacture of such multilayer coatings described in U.S. Pat. No. '005 essentially involves depositing alternating layers of tantala and silica upon the surface of the lamp by low pressure chemical vapor deposition. In order to avoid the-development of catastrophic stresses when the coated lamps are subsequently burned, leading to poor adhesion and poor optical properties, the coated lamps are heat treated to a temperature at least as high as the temperature of the lamp surface when the lamp is burned. Moreover, during this heat treatment process, the temperature of the coated lamp is maintained between 550.degree. and 675.degree. C. for a period of time ranging between 0.5 hour and 5 hours, before being exposed to the higher lamp burning temperature, to control the rate of formation and growth of tantala crystallites during the heat treatment. The higher temperature is applied for 0.1-5 hours, and is at least as high as the lamp surface when the lamp is burned. During the heat treatment process, a pattern of fine randomly oriented cracks develops, resulting in a decrease in the overall, or average, stress. Random cracking is a natural consequence of high stresses in thin films. The heat treatment conditions allow cracked coatings to remain stable during lamp operation.
However, depending upon the coating-deposition and heat-treatment conditions employed in a particular case, the resulting tantala/silica multilayer coatings may be so highly stressed that they begin to peel from the surfaces of the vitreous incandescent lamp envelopes during the heat-treatment process itself. Because it may be difficult or impossible to predict or control the buildup of such unusually high stresses within the multilayer coatings, methods have been sought by which to assure that the tantala-silica coatings remain completely attached to the surfaces of the vitreous substrates during heat-treatment processing.
There is thus a need for an improved method for making a thin film optical coating interference filter upon a vitreous substrate, such as an electric lamp envelope, which coating comprises alternating layers of tantala and silica, which method results in such a coating as is well adhered to the substrate and remains so at temperatures in excess of 600.degree. C. There is further needed a lamp which is provided with such a coating which remains well adhered thereto under operating conditions.